Concept of Demand & Capacity in Structures

Definition of Demand (Loads on structures)

Demand refers to all external actions (self weight) gravity, wind, earthquake, snow all are external actions. If differential settlement occurs stresses are induced in structure. Internal stresses are also called load effects.

Definition of Capacity (Load capacity of structures)

Overall ability of a structure to carry an imposed demand is called capacity.

Analysis of Capacity and Demand

1. To find out whether capacity is enough to carry demands or not.
2. To enhance capacity, either change the material or its geometry.
3. Failure occurs when capacity is less than demand. To avoid failure, capacity to demand ratio should be kept greater than one or at least equal to one.
4. This can be done by increasing the area or reducing the loads or increase the strength of the material by replacing the material
5. Steel Reinforcement area should be opposite to the direction of demand e.g. in slabs the demand direction is in the direction of compression and tension i.e. along the length of bending. So steel is placed along the length and along the width.